This invention relates generally to optical transparencies, and, more particularly, to optical transparencies adapted for use in applications such as in aircraft where they are subject to substantial pressure, temperature and humidity variations in the air spaces on their opposite sides.
Optical transparencies intended for use in aircraft applications, particularly in military aircraft, often are exposed to extreme environmental conditions. The transparency's outside surface frequently is exposed to extremes in pressure, temperature and humidity, yet the air space adjacent the transparency's inside surface must remain suitable for reasonable human comfort. As a consequence, the transparency must be capable of withstanding large pressure, temperature and humidity differentials, all of which can create substantial internal stresses that will tend to bend or warp the transparency out of its desired contour.
Some transparency applications call for at least one of the transparency's surfaces to continuously maintain a prescribed contour, to very precise tolerances. This has sometimes been achieved by providing the transparency with sufficient structural strength to minimize its bending due to the stress factors discussed above. This has generally required the transparency to have a substantial thickness and, thus, substantial weight. In many cases, this weight has been considered excessive. In addition, the transparency bending that can occur even with this substantial thickness and weight can be more than is desired.
It should therefore be appreciated that there is a need for a transparency having an external surface that can maintain its desired contour despite substantial variations in pressure, temperature and humidity in the air spaces on its opposite sides, without the need for excessive structural size and strength and excessive weight. The present invention fulfills this need.